The present invention relates to a method and device for monitoring the performance of internal pumps of a nuclear power plant that utilizes the internal pumps to circulate coolant.
Heretofore, in the field related to a nuclear reactor and the like that creates vapor so as to generate power, a reactor utilizing a jet pump to circulate coolant flow within the reactor and a reactor utilizing an internal pump to do the same are known.
In a jet pump plant, a number of (normally around 20) jet pumps are placed inside the reactor pressure vessel, which are used to send the coolant toward the core so as to cool the core. In this type of nuclear reactor where jet pumps are used to flow the coolant, the flow resistance at each of the various areas of the jet pumps, such as the drive nozzle portion, the suction portion and the throat diffuser, are different according to each jet pump, and further, the portions of the jet pumps can be slightly deformed as time passes. Considering these conditions, the flow resistance of the jet pumps, and therefore the performance of the jet pumps, can vary as time passes.
Japanese Patent Laid-Open Publication No. 6-160582 discloses a method for detecting the performance deterioration of each jet pump by estimating the flow resistance coefficient of each portion of the jet pumps based on the measured flow quantity of the jet pumps, and by monitoring the estimated resistance coefficient. In a jet pump plant, since each diffuser of each jet pump constitutes a flow path, it is possible to monitor the differential pressure and the flow rate variation of each pump by performing an evaluation based on the differential pressure signal equipped to each diffuser.
On the other hand, in an internal pump plant, a large number of (normally around 10) internal pumps are used to send the coolant toward the core, instead of the jet pumps.
FIG. 2 shows an example of a nuclear reactor of an internal pump plant. The internal pumps 111 are mounted in the downcomer portion 113 of a reactor pressure vessel 112, so as to raise the pressure of and send out the coolant flow 114 from the downcomer toward the lower plenum 115. A number of (normally around 10) internal pumps are provided. The flow path of the intake side of the internal pump is shared by all the internal pumps at the downcomer portion, and there are no individual flow paths for each pump as is the case with the jet pump. Therefore, in an internal pump plant, the differential pressure of the pump portion used to compute the pump flow quantity is measured by a differential pressure meter mounted to the upstream side separated from the suction portion of the pumps, so as to enable the use of a common value for each operating pump and so as not to be influenced by the flow near the suction portion of the pump. The pump flow is computed based on a correlation equation measured in advance through a test performed outside the plant, and based on the differential pressure of the pump portion measured as above, the pump speed, and the reactor water temperature.
Since no jet pump is equipped in the internal pump plant, it is impossible to monitor the pump performance by the method disclosed in the above-mentioned publication. In other words, in a jet pump plant, it is possible to mount a differential pressure system to the diffuser of each individual jet pump, and by performing a test for each individual jet pump outside the plant, the relation between the generated differential pressure and the flow rate can be computed in advance for each jet pump, based on which the plant can be monitored during operation.
However, the structure of the internal pump system makes it difficult to mount differential pressure gages to individual pumps, and the measured differential pressure of the pump portion is utilized as a common value for all the pumps. Therefore, if for example the pump performance of one out of ten pumps equipped in the system changes, it is difficult to detect this change in performance as a variation in the differential pressure value or a variation in the pump flow.
The object of the present invention is to provide a method and device for monitoring the performance of internal pumps in an internal pump plant, enabling the variation of the pump performance to be detected at an early stage and also enabling the pump having the varied performance to be specified.
The present invention relates to a method for monitoring the performance of internal pumps of a plant in which the coolant is driven through plural internal pumps, the method comprising measuring the power input to the pump motor that drives the internal pumps, estimating the input power to the pump motor based on the test performed outside the plant corresponding to the pump speed and the flow of the pump measured at the plant, computing the ratio of the actual measured value of the power input to the pump motor to the estimated value, and detecting the performance variation of the internal pump.
According to one example, the ratio of input power to the pump motor for each of the plural number of internal pumps is computed, and when the ratio of input power to the pump motor drops below a threshold set in advance, the pump performance is determined as varied, and the internal pump is specified.
Moreover, the present invention provides a device for monitoring the performance of internal pumps of a plant in which the coolant is driven through plural internal pumps, the device comprising a pump-performance-ratio computing unit for taking in the differential pressure of the pump portion, the pump speed, the pump motor power input of the motor driving the pump, and the pump flow measured at the actual pump, and computing the ratio between the motor power input of the actual pump and the pump motor power input based on the result of the test performed in advance outside the plant, and a pump-performance-ratio display device for displaying the computed input power ratio of the pump motor.